When a high frequency current is driven through a Josephson junction it generates a series of constant voltage steps in its low frequency current versus voltage characteristic. These steps occur at the voltages nf/K.sub.J where n is an integer, f is the microwave frequency and K.sub.J =483,597.9 GHz/V which is the Josephson constant. By international agreement, these steps define the Systeme International d'Unites (SI) volt representation. Series arrays of Josephson junctions are used as voltage standards throughout the world. A typical Josephson voltage standard uses 20,000 or more junctions driven at 75 GHz to generate about 200,000 voltage steps from -14 to +14 volts.
Although these standards can be set to any one of approximately 200,000 steps, the procedure to select a particular step is so slow that the standards are useful only for dc measurements.